LaMotte 1200 Colorimeter - Manual (Page 67)

PHOSPHATE TEST METHOD SPECIFICATIONS
APPLICATION
Drinking, surface and saline waters; domestic and industrial wastes (Method based on
reactions that are specific for orthophosphate).
RANGE
0 to 3.0 ppm Orthophosphate (Range can be extended by dilution.)
METHOD
Ammonium molybdate and antimony potassium tartrate react in a filtered acid medium
with dilute solution of PO
4
3- to form an antimony-phosphomolybdate complex. This
complex is reduced to an intense blue colored complex by ascorbic acid. The color is
proportional to the amount of phosphate present. (Only orthophosphate forms a blue
color in this test.) Polyphosphates (and some organic phosphorus compounds) may be
converted to the orthophosphate form by sulfuric acid digestion. Organic phosphorus
compounds may be converted to the orthophosphate form by persulfate digestion.
HANDLING & PRESERVATION
If benthic deposits are present in the area being sampled, great care should be taken not
to include these deposits. If the analysis cannot be performed the same day of collection,
the sample should be preserved by the addition of 2 mL of concentrated sulfuric acid or
40 mg mercuric chloride per liter and refrigerated at 4°C.
INTERFERENCES
1.
No interference from copper, iron, or silicate at concentrations many times the
concentration of sea water. However, high iron concentrations can cause
precipitation and subsequent loss of phosphorus.
2. Salt error for samples ranging from 5% to 20% salt content was found to be less
than 1%.
3. Mercuric chloride, HgCl
2
, when used as the preservative, interferes when the
chloride levels are low (less than 50mg/L). This interference is overcome by spiking
samples with a minimum of 50 mg/L of sodium chloride.